Electric lamp circuits



y 6, 1954 J. w. STRANGE ET AL 8 0 ELECTRIC LAMP CIRCUITS Filed Sept. 20,1949 3 Sheets-Sheet 1 J. w. STRANGE ET AL 2,683,240

July 6, 1954 ELECTRIC LAMP CIRCUITS- 3 Sheets-Sheet 2 Filed Sept. 20,1949 I I v H 1/ 1 T 2 R '5 1., T3 2 1 "r, 'L/ 1- T, L2, 7 T

I W3 g?!- T g T6 5,? 4 g a fi w I a m W WI E EJH 8 INVENTOR5:

K/OHIY MAL-LIAM STRANGE DOUGLAS THO/mu WAIGH WILLIAM ROBERT BLOXSIDGE A710R zy,

y 1954 J. w. STRANGE ET AL 2,683,240

ELECTRIC LAMP CIRCUITS Filed Sept. 20, 1949 a Sheets-Sheet 3 I w oooooooPatented July 6, 1954 ELECTRIC LAMP CIRCUITS John William Strange,Douglas Thomas Waigh,

and William Robert Bloxsidge, London, England, assignors to ThornElectrical Industries Limited, London, England, a British companyApplication September 20, 1949, Serial No. 116,756

3 Claims.

This invention relates to circuits used to control the brightness oflow-pressure discharge lamps of the hot-cathode type. A low-pressuredischarge lamp is one which operates normally at a temperature whichexceeds the ambient temperature by not more than 60 C.

Filament lamps may be fed through adjustable dimming means capable ofreducing the light emission progressively to zero. If coloured light isrequired as, for example, in theatre lighting, 1'

colour filters can be fitted in front of the filament lamps. Theefficiency of the combination of filament lamp and colour filter is verylow. The use of fluorescent discharge lamps has been suggested forcolour lighting, since, by suitable choice of the luminescent material,a wide range of colours can be produced without the use of filters. Thedisadvantage of such lamps is that with normal circuits, consisting of alamp in series with a choke and the filamentary lamp electrodesconnected in series for starting by a glow or thermal switch, only alimited range of brightness is available. Attempts to use low voltagesresult in erratic and intermittent operation.

An object of this invention is to provide brightness control means whichwill permit substantial dimming of low-pressure discharge lamps of thehot-cathode type, and especially fluorescent lamps.

Another object of the invention in some of its forms is to provide anelectric lamp system which can be arranged to give a substantiallysmooth decrease of brightness over the range from full brightness toless than one-hundredth continuously to each of the electrodes of thelamp at least over the lower part of the operating range of brightnessof the lamp, and means for applying between the terminals of the saidcombination a potential difference which is substantially continuouslyvariable by a dimmer control member while the lamp is operating,

both said means being adapted to be supplied from a common source ofpower. According to the invention in a preferred particular aspect, thesaid means for applying the potential difference which is variable whilethe lamp is operating are a variable series impedance element. Thisimpedance element may be a variable resistor or a variable saturablereactor, or a variable resistor in series with a variable saturablereactor.

A filament lamp may be connected in parallel with the discharge lamp,the arrangement being such that, in the operating range within which thepotential difference across the lamps is insufficient to cause thedischarge lamp to strike, an appreciable amount of light is emitted bythe filament lamp.

There may be provided a lamp system including two or more of the lampcircuits hereinbefore specified as according to this invention connectedin parallel and having a common dimmer control member.

Two further particular aspects of this invention and three separatearrangements of the third aspect will now be specified.

According to this invention in its second aspect, an electric lampcircuit includes a lamp and ballast combination consisting of twolowpressure electric discharge lamps of the hotcathode type connected inseries to form a single discharge path, and a ballast impedance elementadapted to limit the total current flowing in the discharge path, meansfor supplying heating current continuously to each of the electrodes ofthe lamps of the said combination at least over the lower part of theoperating range of brightness of the lamps, and also means for providingbetween the terminals of the said combination a potential difierencewhich is substantially continuously variable by a dimmer control whilethe lamps are operating, both said means being adapted to be suppliedfrom a common source of power, and an impedance element connectedbetween the junction of the discharge spaces of the lamps and a point onthe circuit, e. g. a terminal of the source, so selected that thelastmentioned impedance element will pass a current through at least oneof the lamps. The magnitude of this current should be such as tomaintain the discharge at the lower limit of the controllable range ofbrightness of the lamp. A third lamp may be connected in series with theother two lamps, in which case a further impedance element may beconnected between the junction of the third lamp with the other twolamps and a point on the circuit, e. g. the other terminal of thesource, so selected that the lastmentioned impedance element will passthrough the said third lamp a current sufficient to maintain thedischarge at the lower limit of the controllable range of brightness ofthe lamp. The said means for supplying heating current may include atransformer having itsprirnary winding connected either across thesupply terminals or in parallel with the discharge path. There may beprovided a lamp system including two or more lamp circuits, specified asaccording to the invention in its second aspect, these circuits beingconnected in parallel, and the said potential diiierence, which isapplied to the lamp and ballast combination referred to above, beingsubstantially continuously variable by meansoi a common dimmer controlmember, e. grasseciated with a common dimmer element of the kind whichvaries the applied voltage, such'as a variably-tapped autotransiormer.

According to this'invention inits third aspect an electric lamp circuitincludes a lamp and ballast combination consisting of a low-pressureelectric discharge lamp of the hot-cathode type having two electrodes,and a ballast impedance element connected in series withthe lamp andadapted to limit the total current flowing between the electrodes, meansfor applying between the terminals of the said lamp and ballast combination a first potential difference which is substantiallycontinuously variable by a dimmer control while the lamp is operating,and means for applying between the terminals of each electrode a secondpotential difference which issubstantially continuously variable by saiddimmer control up to a predetermined value-while a relatively low valueof saidfirst potential difierence (i. c. lower than the valueofthestriking voltage of the lamp) is applied between the lampelectrodes, and for preventing said second potential difference fromsubstantially exceeding said predetermined value while said firstpotential difference is increased beyond aid relatively low value.

There may be provided a lamp system including two more lamp circuits,specified as according to the invention'in its third aspect, connestedparallel and having a common dimmer control.

In a first arrangement according to the invention in the said thirdaspect, an electric lamp circuit includes a lamp and ballast combinationconsisting of a low-pressure electric discharge lamp of the hot-cathodetype having two electrodes, and a ballast impedance adapted to limit thetotal current flowing between the electrodes, dimmer means for providingbetween the terminals of the said combination a potential differencewhich is substantially continuously variable by a dimmer control memberwhile the lamp is operating, and means for supplying to each of theelectrodes of the lamp heating'current which is substantiallycontinuously variable by a regulator while a relatively low value of'potential difference is applied between the said electrodes, both meansbeing adapted to be suppliedfrom a common source of power. fhe dimn ercontrol member and the regulator may be ganged. There may be provided alamp system including two or more lamp circuits specified as said firstarrangemer having a common regulator and a common dimmer control memberactuating a dimmer element, e. g a variably-tapped autotransformer, ofthe kind which varies the'applied voltage.

ages between the terminals of a lamp and ballast combination consistingof a low-prcscure electric discharge lamp of the hot cathode type, and aimpedance connected in series with the lamp and adapted to limit thetotal current flowing between the lamp electrodes, element also beingconnected to feed throng capacitor the primary winding of a secondtransformer having separate secondary windings connected respectivelyacross the lamp electrodes, and the arrangement being such that for avalue of the potential difference applied between the lair-p electrodesslightly below the value of the stri ng voltage of the lamp the-circuitis resonant at the frequency of the power source.

There may be provided a lamp system including two or more lamp'circuitsspecified as said second arrangementconnected inparallel and having acommon dimmerelement.

In a third arrangement according the invention in the said third aspect,an electric lamp circuit for use with an A. 0. power source includes adimmer element-in the form of a variably-tapped autotransiormerconnected between the terminals of the power source and adapted toprovide substantially continuously variable voltages between theterminals of a and ballast combination consisting of a low-pressuredischarge lamp of the hot-cathode type connected in series with aballast impedance element which is adapted to limitthe total currentflowing between the lamp electrodes, said dimmer element also beingconnected to feed the primary winding of a voltage limiting transformer,and the circuit also including a second transformer having a primarywinding supplied through the secondary winding of said voltage limi"ngtransiormer and separate secondary windings connected respectivelyacross the lamp electrodes.

There may be provided a lamp system including two or more lamp circuitsspecified as said third arrangement, connected in parallel and having acommon dimmer element and a com mon voltage limiting transformer.

In all the above-mentioned circuits, when supplied from A. C, powersource, the means for supplying heating current to the lamp electrodesconsist preierably of a transformer of the leakage-field reactance type.

In all the above-mentioned circuits an external electrode adapted to bemaintained at a fixed potential is preferably placed close to thedischarge lamp envelope, at least in the nei hbourhood of the lampelectrodes, in order to increase the ionisation of the spaces adjoiningthese elec trodes before tl e lamp strikes.

Preferred circuit arrangements embodying the invention will now bedescribed, by way of example, with reierence to the accompanyingdrawings, each iigure of which shows a lamp system for use with an A. C.power source and consisting of two lamp circuits connected in parallel.The mains voltage is 230 unless otherwise stated. In the drawings:

Figure 1 shows a lamp system according to the invention in the saidfirst aspect and in which each lamp circuit includes two discharge lampshaving continuously heated electrodes and connected in series to form asingle discharge path; the circuits have separate adjustable electricalcomponents acting as dimmers and ganged for operation by a commoncontrol member.

Figure 2 shows a lamp system according to the invention in which eachlamp circuit includes a single discharge lamp having continuously heatedelectrodes; the circuits have a common adjustable electrical. elementacting as a dimmer.

Figure 3 shows a lamp system illustrating the invention in the saidsecond aspect and in which the lamp circuits are provided with a commonadjustable electrical. element acting as a dimmer.

Figure 4 shows a lamp system similar to that shown in Figure 3 exceptthat the lamp circuits have separate but ganged adjustable electricalelements acting as dimmers.

Figure 5 shows a lamp system illustrating the adjustable electricalelements acting respectively as a dimmer and as a regulator.

Figure 6 shows a lamp system similar to that shown in Figure 5 exceptthat each lamp circuit includes a single discharge lamp instead of twodischarge lamps connected in series.

Figure 7 shows a lamp system illustrating the second arrangement of theinvention in its third aspect; the lamp circuits have a commonadjustable electrical element acting as a dimmer.

Figure 8 shows a lamp system similar to that shown in Figure 7 exceptthat each lamp circuit includes a single discharge lamp instead of twodischarge lamps connected in series.

Figure 9 shows a lamp system illustrating the third arrangement of theinvention in its third aspect; the lamp circuits have a commonadjustable electrical element acting as a dimmer.

Figure 10 shows a lamp system similar to that shown in Figure 9 exceptthat each lamp circuit includes a single discharge lamp instead of twodischarge lamps connected in series.

In each figure of the drawings elements in one lamp circuitcorresponding to like elements in the other lamp circuit will be givenlike refer ence characters. All the lamps in the circuits dc.- scribedare of the type having directly heated electrodes.

Referring now to Figure 1, which illustrates the invention in its firstaspect, in each lamp circuit two discharge lamps L1 and L2 havingfilamentary electrodes E1, E2 and E3, E4. respectively are connected inseries to form a single discharge path. The terminals of the filamentaryelec trodes are indicated by T (e. g. T1, T2, T3, etc.). The lamps areconnected, in series with a ballast choke B and a dimmer elementconsisting of a variable resistor R1, between terminals L and N of an A.C. power source. A transformer F1, of the leakage-field reactance type,has a primary winding W1 connected between the source terminals andseparate secondary windings W2 and W4 connected to supply heatingcurrent to electrodes E1 and E4 respectively. Electrodes E2 and E3 areconnected in series with a further separate secondary winding W3 whichsupplies them with heating current. External electrodes S1 and S2consisting of metallic strips connected to earth are disposed alongsidethe envelopes of their respective lamps L1 and L2. The dimmer elementsare ganged and connected to a common dimmer control member indicateddiagrammatically by P.

Each transformer F1 is so designed that the electrodes of the lampsheated by the current from the secondary windings W1, W2 and W: are keptat such a temperature that they emit electrons freely. The presence ofthe adjacent earthed strips encourages ionisation around the electrodes.A stable discharge can be passed between the two electrodes of each lampwith the control set to limit the current to a value as low as 2milliamperes. As the series impedance is decreased, by operation of thedimmer control, the current passing through the lamps increases untilthe lamps reach full brightness.

Thus the brightness of the lamps can be varied from their maximum to alow value without instability by operation of the resistors R1. Thegauging of these resistors enables the brightness of all the lamps inthe system to be varied substantially simultaneously by actuation of asingle operators control.

This circuit arrangement is particularly suitable for a fluorescent lamphaving an envelope 2 ft. in length and 1 /2 in. in diameter, designed torun with a lamp wattage of 4.0. A suitable value for each of theresistors R1 is 130,600 ohms. Measurements made on such a circuit, withlamps having a luminescent coating of manganese activated zinc silicate,show that the light emitted by each lamp increases from 5 to 1300lumens, reaching a peak eiiiciency of L./watt. Any number of similarlamp circuits may be connected in parallel and their dimmer resistorsganged.

A transformer of the lealragafield reactance type is selected for F1since in operation the lamp electrodes have a negative impedancecharacteristic. This negaitve impedance characteristic is due to smallpotential differences which exist between adjacent portions of theelectrode and which cause the establishment of minor discharges betweenthese adjacent portions.

The arrangement shown in Figure i may be modified by replacing thevariable dimmer resisters R1 by variable saturable reactors.Alternatively each dimmer element may consist of a variable resistor inseries with a variable saturable reactor; this arrangement canconveniently be made such that the reactor controls the brightness fromfull brightness to, say, one-tenth full brightness, the resistorcontrolling the brightness over the remainder of the range.

In another modification or" the arrangement shown in Figure l, in eachlamp circuit the two lamps L1 and L2 are replaced by a 5-foot Bil-wattlamp, the transformer winding W3 being omitted. The circuit is designedfor operation on 24 0 volts, and a filament lamp, designed to operate ona lSO-volt supply at watts, is connected in parallel with the dischargelamp. With this arrangement, increase of the potential difierence acrossthe lamps will increase the brightness or" the filament lamp until thestrike potential of the discharge lamp is reached, whereupon thepotential difference across the lamps immediately decreases, so that asubstantailly smooth increase in the brightness of the two lamps incombination is achieved through this transition period.

In each of the circuits in the system shown in Figure 2 a single lamp L1is connected in series with a ballast choke 33 between terminals T9 andT10 of a variably-tapped secondary winding We of a transformer F2 whichacts as a dimmer control and which is common to the two circuits. Theterminals T11 and T12 of the primary winding W5 of the transformer F2areconnected respeotively to source terminals N and L. A leakage-fieldreactance transformer F1 has a primary winding W1 connected between thesource termi male and separate secondary windings W2 and W4 connected tosupply heating current to lamp electrodes E1 and E respectively. Anexternal electrode S1 is provided.

The brightness of the lamps may be varied by operating the dimmercontrol member and thus altering the potential applied to the lamp andthe ballast choke in series. A discharge lamp, when in the dischargingcondition, has the characteristic of a constant voltage device, andvariations in the potential applied to the lamp and the ballast choke inseries will cause comparatively small variations in the potentialdiirerence between the lamp electrodes.

This circuit arrangement is particularly suit able for a fluorescentlamp having an envelope 5 ft. in length and 1 in. in diameter, designedto run with a lamp wattage of 80. Any number of similar lamp circuitsmay be connected in parallel provided that the power rating of thedimmeitransformer F2 is increased to a sufficient value.

In the system shown in Figure 3, illustrating the invention in itssecond aspect, each circuit includes two discharge lamps L1 and L2, aballast choke B and a leakage-field reactance transformer F1 connectedin the manner described with reference to Figure 1. External electrodesS1 and S2 are provided. The lamp and ballast combination is connectedbetween terminals T14 and T of winding W 7 of a variably-tappedautotrans former F2 acting as the dimmer. and T14 of the dimmer areconnected to terminals N and L of the power source respectively. T10 isthe terminal of the tapping which constitutes the dimmer control. Thejunction of terminals T3 and T5 or" electrodes E2 and E3 respectively isconnected to the power source terminal N through a second impedanceelement consisting of a resistor R2 in series with a capacitor C1. Thevalues or" R2 and C1 are such that a small glow discharge, representingthe lower limit of the controllable range of brightness, is maintainedin either or both of the lamps. When the source terminals are phase andneutral, the second impedance element is preferably connected to theneutral terminal. trol is in its position of minimum brightness, atleast one of the lamps of each pair (usually L2) is in or near thecondition of small glow discharge and all the lamps will begin toincrease in brightness at substantially the same rates when the dimmercontrol is moved from this minimum brightness position. Therefore asingle operators control may be used despite variations from lamp tolamp of the potential normally required to cause each lamp to strike.

This circuit 'angement is particularly suitable ior Z-foot 40-wattlamps, and suitable values or R2 and 01 for use with such lamps are100,000 ohms and 5,000 pf. respectively.

in a modification of the arrangement shown in Figure 3, the capacitorsC1 are connected to the tapping terminal T10 instead of to the new tralterminal N.

Any number of similar circuits may be connected in parallel providedthat a separate second impedance element is provided for each pair oflamps and that the rating of the dimmer transformer is increased to asuitable value.

The lamp system shown in Figure 4 is similar to that shown in Figure 3except that the dim- Terminals T13 When the dimmer connier for eachcircuit consists of a resistor R1 connected in series with the lamp andballast combination between the power source terminals. These separateresistors R1 have their tappings ganged by means indicateddiagrammatically by P to enable the brightness of all the lamps to bevaried substantially simultaneously by a single operators controlmember. This circuit arrangement is also particularly suitable for2-foot 40- watt lamps. A suitable value of R1 is 130,000 ohms.

In the system hereinbefore described the lamp electrodes heatedcontinuously andwhen the power applied to each lamp is r duced below thelevel at which a column discharge is visible, the ends of the lamp willstill glow owing to the ionisation of the spaces surrounding the hotelectrodes. If it is desired to extinguish the lamp completely theelectrode heating currents may be switched off but the desired smoothdecrease of brightness will not then be obtained. Circuits in which thelamps can be completely extinguished will now be described.

The arrangement shown in Figure l may be modified by eliminating theconnections shown for each primary winding W1 and connecting theterminals of each of these windings directly to the terminals T1 and T7of the associated lamps.

The lamp system shown in Figure 5, illustrating the first arrangement ofthe invention in its third aspect, is similar to the system shown inigure 3 except that the primary windings W1 or" leakage-field reactancetransformers F1 are connected in parallel between terminals T15 and T17Of winding We of a variably-tapped autotransformer F3 connected as aregulator. Thus the values of the electrode heating currents may also bevaried. Also the second impedance element (i. e. the resistor R2 and thecapacitor C1) is connected to the terminal T16 of the regulator insteadof to the terminal N of the source, thereby enabling the lamps to becompletely extinguished when the dimmer control. and the regulator areset to zero.

The dimmer control and regulator transformers F2 and F3 are ganged bymeans of levers A operatively connected to the tapping selectors G andpivoted at C. The levers are moved by cams J and push-rods K and areheld in contact with the push-rods by compression springs H. The camsare mounted on a shaft M rotatable in bearings V. An operators handle 0is also mounted on shaft M. The relative rates of movement of thetapping selectors of the dimmer and the regulator transformers may bealtered in known manner by varying the cam contours.

This circuit arrangement is particularly suitable for 2-foot iii-wattlamps. In order to assist the ionisation of the spaces surrounding theelectrodes before the arc strikes, the minimum value of the potentialdifference applied across each pair of lamps is set at approximatelyvolts. This potential difference is applied while the regulator isoperated to increase the electrode heating currents to their maximumvalues, causing the ends or the lamps surrounding the electrodes tobegin to glow and eventually causing the lamps to strike; the electrodeheating current is then maintained substantially constant while theclimrner control is operated to increase the potential difference acrossthe lamps until they have reached full brightness. To extinguish thelamps the sequence of operation is reversed. Thus it is possible toincrease the brightness of all of the lamps at substantially the samerate over the full range of zeroto maximum brightness using a singleoperators control.

Any number of similar circuits may be connected in. parallel providedthat the ratings of the dimmer and the regulator transformers areincreased to suitable values.

The system shown in Figure 6 is similar to that shown in Figure exceptthat a single lamp is used instead of two lamps connected in series.This circuit arrangement is particularly suitable for Bil-watt lamps.

The system shown in Figure 7, illustrating the second arrangement of theinvention in its third aspect, is similar to the system shown in Figure3 except that the primary windings W1 of transformers F1 are connectedin series with capacitors C2 and that each capacitor C2 and itsassociated primary winding W1 are connected in parallel with the lampand ballast combination, The value of the capacitor C2 is such that whenthe tap of the autotransformer F2 is set to apply, between the terminalsof the lamp and ballast combination, potential difference which isslightly below the striking voltage of the lamps, the circuit isresonant at the frequency of the power source.

When the dimmer control is operated to increase the potential difierenceapplied to the lamp and ballast combination, the heating current flowingthrough each electrode is also increased. As the sub-circuit includingthe transformer F1 approaches the resonant condition, its impedancedecreases until at resonance its impedance reaches a minimum. Thecurrent flowing through the primary of the transformer F1 and theheating current flowing through the electrodes will therefore be ofmaximum value. The potential difference applied across the lamps assiststhe ionisation of the spaces surrounding the electrodes and thisionisation causes the ends of the lamps to glow. Progressive operationof the dimmer control increases the potential difference applied to thelamp and ballast combination until the lamps are at full brightness.When the dimmer control is so operated, the sub-circuit tends to go outof resonancaits impedance increases and the electrode heating currentstherefore decrease. This decrease is compensated by the heating eifectof the discharge which is now established.

This circuit arrangement is particularly suitable fcr 2-foct ib-wattlamps. Any number of similar circuits may be connected in parallelprovided that the rating of autotransformers F2 is increased to asuitable value and that a separate capacitor is provided for eachprimary winding W1 of transformer F1. When used with a 230- volt,50-cycle power source, a suitable capacitor had a value of 2 mi. and wasrated for a maximum working voltage of 440 volts A. C.

The system shown in Figure 8 is similar to the system shown in Figure 7except that each circuit includes a single lamp instead of two lampsconnected in series. This circuit arrangement is particularly suitablefor EEO-watt lamps.

The system shown in Figure 9 illustrating the third arrangement of theinvention in its third aspect is similar to the system shown in Figure 3except that the primary winding W1 of each transformer F1 is connectedbetwen output terminals T18 and T19 of a voltage limiting transformerF4. The input termials T20 and T21 of the transformer Fr. are connectedto terminals T and T14 of the dimmer transformer F2. Also the secondimpedance element (i. e. resistor R2 and capacitor C1) is connected tothe terminal each reached a value sufiicient to enable the lamps tostrike. Under these conditions ionisation occurs around the electrodesandthe ends of the lamps glow. Further operation of the dimmer controlincreases the potential difference between the lamp electrodes and thelamps strike, whilst the electrode heating currents are maintainedsubstantially constant by the voltage limiting transformer. The dimmercontrol may then be operated to bring the lamps up to full brightness.Such a voltage limiting transformer is well known to those skilled inthe art.

This circuit arrangement is particularly suitable for 2-foot lo-wattlamps. Any number of similar circuits may be connected in parallelprovided that the ratings of voltage limiting transformer F4 and dimmertransformer F2 are increased to suitable values.

The system shown in Figure 10 is similar to that shown in Figure 9except that each circuit includes a single lamp instead of two lampsconnected in series. This circuit arrangement is particularly suitablefor -watt lamps.

In the circuit arrangements in which variablytapped transformers areused these transformers are so connected that the full mains potentialis kept between the lamps and the external electrodes S irrespective ofthe positions of the transformer taps. With such an arrangement smoothvariation in the brightness of the lamps is more readily obtained.

In all the circuit arrangements, when the terminals of the source arerespectively phase and neutral, the ballast impedance is preferablyconnected between the lamps and the phase terminal.

We claim:

1. An electric lamp circuit including two power terminals for connectionto an alternating cur rent power source of fixed frequency and voltage,two electric discharge lamps and a ballast impedance connected in seriesbetween said power terminals, each of said discharge lamps having afilamentary cathode at each end thereof; means for supplying heatingcurrent to each of said filamentary cathodes; means for reducing thecurrent to said lamps to dim the same; and a capacitative impedanceconnected in series with said ballast impedance and one only of thelamps between said power terminals, said capacitative impedance beingsmall enough to pass enough current through the lamp in series with itto maintain the discharge therethrough at the lower limit of brightnessof the lamp.

2. An electric lamp circuit including two power terminals for connectionto an alternating current power source of fixed frequency and voltage,two electric discharge lamps and a ballast impedance connected in seriesbetween said power terminals, each of said discharge lamps having afilamentary cathode at each end thereof; means for supplying heatingcurrent to each of said filamentary cathodes; and a capacitativeimpedance connected in series with said ballast impedance and one onlyof the lamps between said power terminals, said capacitative impedancebeing small enough to pass enough current through the lamp in serieswith it to maintain the discharge therethrough at the lower limit ofbrightness of the lamp.

3. An electric lamp circuit including an alternating current powersource of fixed frequency and voltage, two electric discharge lamps anda ballast impedance connected in series between said power terminals,each of said discharge lamps having a filamentary cathode at each endthereof; means for supplying heating current to each of said filamentarycathodes; and a capacitative impedance connected in series with saidballast impedance and one only of the lamps be tween said powerterminals, said capacitative impedance being small enough to pass enough20 current through the lamp in series with it to maintain the dischargetherethrough at the lower limit of brightness of the lamp.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,481,876 Mutscheller Jan. 29, 1924 1,977,231 Erickson Oct.16, 1934 2,056,661 Foulke Oct. 6, 1936 2,392,845 Foerste Jan. 15, 19462,438,564 Lemmers Mar. 30, 1948 2,458,277 Lark Jan.'4, 1949 2,487,092Bird Nov. 8, 1949 2,523,021 Hinman Sept. 19, 19.50

FOREIGN PATENTS Number Country Date 623,199 Great Britain May 13, 1949

